The present invention improves thermal uniformity of a workpiece being heated in a fixed coil induction heating workcell of the kind described in Boeing's U.S. Pat. No. 5,624,594. The workcell is useful for processing metals or resin composites, especially in combined cycle operations that greatly reduce processing time. These operations achieve the best results when the operating temperature is uniform in the workpiece. For simplicity, for the remainder of this description we will refer to manufacturing problems with reference to composite processing. The manufacturing problems associated with metal processing, like SPF, brazing, and the like, are comparable to those for composite manufacturing, if not more significant, because even higher temperatures are required.
1. Composite Manufacturing
Fiber-reinforced organic resin matrix composites typically have a high strength-to-weight ratio and a high stiffness-to-weight ratio. Desirable fatigue characteristics, especially for sandwich panels, make them increasingly popular as a replacement for metal in aerospace applications where weight, strength, or fatigue is critical. Manufacturing these thermoplastic or thermoset composites is expensive today. Improved manufacturing processes would reduce their cost by reducing touch labor and forming time.
Prepregs combine continuous, woven, or chopped reinforcing fibers with an uncured, matrix resin, and usually comprise woven or unidirectional fiber sheets with a thin film of the matrix. Sheets of prepreg generally are placed (laid-up) by hand or with fiber placement machines directly upon a tool or die having a forming surface contoured to the desired shape of the completed part or are laid-up in a flat sheet which is then draped and formed over the tool or die to the contour of the tool. The resin in the prepreg lay up usually is consolidated (i.e. pressed to remove any air, gas, or vapor) and cured (i.e., chemically converted to its final form, usually through chain-extension) in a vacuum bag process in an autoclave (i.e., a pressure oven) to complete the part.
The tools or dies for composite processing typically are formed to close dimensional tolerances. They are massive, must be heated along with the workpiece, and must be cooled prior to removing the completed part. The delays caused to heat and to cool the mass of the tools adds substantially to the overall time necessary to fabricate each part. These delays are especially significant when the manufacturing run is small and at a low rate. In these circumstances, the dies need to be changed frequently, often after producing only a few parts of each kind. Change out of the dies requires the long times associated with heating and cooling. An autoclave has similar limitations; it is a batch operation.
In hot press forming, the prepreg is laid-up to create a preform, which is bagged (if necessary), and placed between matched metal tools that include forming surfaces to define the internal, external, or both mold lines of the completed part. The tools and composite preform are placed within a press and then the tools, press, and preform are heated.
The tooling in autoclave or hot press fabrication is a significant heat sink that consumes substantial energy. Furthermore, the tooling takes significant time to heat the composite material to its consolidation temperature and, after curing the composite, to cool to a temperature at which it is safe to remove the finished composite part. So, the tooling is expensive to manufacture and expensive to use because of the relatively long processing times increased with heating and cooling of the tooling.
Actively cooling the tools after forming the composite part has shortened the time necessary to produce a composite part, but the cycle time for and cost of heating and cooling remains significant to overall fabrication cost. Designing and making tools to permit their active cooling increases the cost of the tools.
Boeing described a process for organic matrix forming and consolidation using induction heating in U.S. Pat. No. 5,530,227. There, prepregs were laid up in a flat sheet and were sandwiched between aluminum susceptor facesheets. The process was similar to the "double diaphragm" forming process described in U.S. Pat. No. 4,657,717, except that the facesheets were susceptible to heating by induction and formed a retort to enclose the prepreg preform. To ensure an inert atmosphere around the composite during curing and to permit withdrawing volatiles and outgassing from around the composite during the consolidation, the facesheets were welded together around their periphery. Welding added preparation time and increased the cost for part fabrication by adding labor and material cost. Welding ruined the facesheets and prohibited their reuse which added a significant cost penalty to each part. In U.S. Pat. No. 5,599,472, a technique reliably seals facesheets of the retort without the need for welding and permits reuse of the facesheets in certain circumstances.
2. Processing in an Induction Press
Boeing uses ceramic dies for induction processing because a ceramic is not susceptible to induction heating and, preferably, is a thermal insulator (i.e., a relatively poor conductor of heat). Additional reinforcement can be added, if desired, as described in U.S. Provisional patent application Ser. No. 60/071,765. Cast ceramic tooling is strengthened and reinforced internally with fiberglass rods or other appropriate reinforcements and externally with metal or other durable strongbacks. The reinforcement permits the dies to withstand the temperatures and pressures necessary to form, to consolidate, or otherwise to process the composite materials or metals. Cast ceramic tools cost less to fabricate than metal tools of comparable size. We embed induction heating elements in the ceramic tooling to heat the composite or metal retort without significantly heating the tools. The induction heating elements form a water cooling network within the induction coil. Thus, induction heating can reduce the time required and energy consumed to fabricate a part.
While graphite or boron fibers can be heated directly by induction, most organic matrix composites require a susceptor in or adjacent to the composite material preform to have the band line uniformly for consolidation or forming, without causing other problems with the laminates. The susceptor is heated inductively and transfers its heat principally through conduction to the preform or workpiece that, in our prior work, is sealed within the susceptor retort. Enclosed in the metal retort, the workpiece does not experience the oscillating magnetic field which instead is absorbed in the retort sheets. Heating is by conduction from the retort to the workpiece.
Induction focuses heating on the retort and workpiece and eliminates wasteful, inefficient heat sinks in the tooling. Because the ceramic tools in our induction heating workcell do not heat to as high a temperature as the metal tooling of conventional presses, problems caused by different coefficients of thermal expansion between the tools and the workpiece are reduced. A significantly higher percentage of our input energy goes to heating the workpiece than occurs with conventional presses. Our reduced thermal mass and ability to focus the heating energy permits us to change the operating temperature rapidly which improves the products we produce. Finally, the factory is not heated as significantly from the radiation of the large thermal mass of a conventional press, and is a safer and more pleasant environment for the press operators.
In induction heating for consolidating and/or forming organic matrix composite materials, we place a thermoplastic organic matrix composite preform of PEEK or ULTEM, for example, within a metal susceptor envelope (i.e., retort). These thermoplastics have a low concentration of residual volatile solvents and are easy to use. The susceptor facesheets of the retort are inductively heated to heat the preform. Consolidation and forming pressure consolidate and, if applicable, form the preform at its curing temperature. The sealed susceptor sheets form a pressure zone. We evacuate the pressure zone in the retort in a manner analogous to conventional vacuum bag processes for resin consolidation. For resins like ULTEM polyimide that include a smaller amount of volatiles, we can pressurize this zone to enhance consolidation. The retort is placed in an induction heating press on the forming surfaces of dies having the desired shape of the molded composite part. After the retort and preform are inductively heated to the desired elevated temperature, we apply differential pressure (while maintaining the vacuum in the pressure zone around the preform) across the retort. The retort functions as a diaphragm in the press to form the preform against the die with controllable tooling pressure into the desired shape of the completed composite panel.
The retort often includes three susceptor sheets sealed around their periphery to define two pressure zones. The first pressure zone surrounds the composite panel/preform or metal workpiece and is evacuated and maintained under vacuum. The second pressure zone is pressurized (i.e., flooded with gas) at the appropriate time and rate to help form the composite panel or workpiece. The shared wall of the three layer sandwich that defines the two pressure zones acts as the diaphragm in this situation.
Boeing performs a wide range of manufacturing operations in its induction heating press at operating temperatures ranging from about 350.degree. F. (175.degree. C.) to about 1950.degree. F. (1066.degree. C.). For each operation, it is necessary to hold the temperature relatively constant for several minutes to several hours. Temperature control by controlling the input power fed to the induction coil is too crude. A better and simpler way capitalizes on the Curie point. By judicious selection of the metal or alloy in the retort's susceptor facesheets, we can avoid excessive heating irrespective of the input power. The improved control and improved temperature uniformity in the workpiece from the Curie point materials produces better products. The Curie point control method is explained in greater detail in U.S. Pat. No. 5,728,309. The Curie point control method controls the absolute temperature of the workpiece by matching the Curie point of the susceptor to the desired temperature of the induction heating operation being performed.
The Curie point improvement sometimes still fails to obtain substantial thermal uniformity in the workpiece. At the ends of the coil, the magnetic flux does not enter the workpiece uniformly to produce uniform heating. Hot spots can occur at the ends, or the ends can reach the Curie point faster than the middle of the workpiece, or vice versa. At the ends, the magnetic flux density in the air or ceramic die surrounding the workpiece is not uniform in part because this surrounding medium is not magnetic.